1. Field of the Invention
The present invention relates in general to an exhaust emission control system of an internal combustion engine, and more particularly, to a control system for controlling air-fuel ratio of a combustible mixture fed to an internal combustion engine.
2. Description of the Prior Art
Hitherto, for controlling the air-fuel ratio of a combustible mixture fed to an internal combustion engine, a so-called "jump back control system" has been widely employed in which a non-linear output signal of an oxygen sensor, which undergoes a sudden change in its output at the stoichiometric air-fuel ratio of the combustible mixture, is used with no modification thereof for controlling the air-fuel ratio. This will be understood from the performance curve "gl" (the curve illustrated by a dot-dash line) of FIG. 2, which is the non-linear output signal issued from the oxygen sensor. In the jump back control system, the output signal from the oxygen sensor is converted into a binary signal, and the fuel injection time is controlled in accordance with an instruction signal, consisting of a proportional part and an integral part, which is based on the binary signal. Japanese Patent First Provisional Publication No. 61-10762 shows one of the conventional control systems of the above-mentioned type.
However, due to its inherency in construction, the above-mentioned type control system has an occasion wherein a control error at the normal operation mode of the engine becomes considerable due to both the sudden change of the output of the oxygen sensor and the inevitable response delay of the air-fuel mixture injecting timing with respect to the time when the exhaust gas is detected by the oxygen sensor. In fact, as is seen from FIG. 8, a considerable response delay appears in the exhaust system of an internal combustion engine. FIG. 6B is a graph which depicts an air-fuel ratio under a condition wherein the frequency of the air-fuel ratio control system is 1 Hz and the engine runs at a speed of 3000 rpm with no load applied thereto. As is seen from this graph, a control error in the range of the air-fuel ratio from 0.6 to 0.7 is produced. This error causes lowering in the purifying efficiency of a three-way catalytic converter installed in the exhaust system. In order to eliminate this drawback, a larger amount of catalysts is usually necessary. However, this measure causes increase in production cost of the converter.